Municipal solid waste (MSW) facilities incinerate trash and garbage in furnaces at temperatures of up to about 2500 degrees F. In order to recover the valuable energy produced in these MSW plants, water is passed through metallic waterwall tubes adjacent to the furnace and converted to steam by the high temperatures. A conventional waterwall boiler tube assembly comprising metallic tubes T connected by membrane M is provided in FIG. 1. The steam produced in the tube assembly is then used to power a turbine-driven electrical generator. However, the MSW plant also produces gaseous products which, if allowed to contact the metal tubes, would chemically attack those tubes. To prevent direct attack of the tubes by the gaseous products and still allow the tubes to be sufficiently heated, a protective refractory lining is placed between the waterwall tubes and the furnace fireside.
Although these refractory linings help to minimize attack on the metallic tubes, their use inhibits the heat flow from the furnace fireside to the waterwall tubes. Maximum heat flow is critical to achieving boiler efficiency. If the refractory lining has insufficient heat transfer the fireside surface of the refractory becomes hotter than designed. As the temperature increases, ash from the fuel being burned will cling to the surface and form an insulating layer. Once this phenomenon begins, the layer gets increasingly thick until heat transfer becomes extremely poor. The "flue gas" above the combustion zone then increases in velocity and temperature, often above the design limits, and causes corrosion/erosion problems downstream in the furnace. In addition, the layer of ash/slag buildup may eventually break off as it grows and cause major damage to the stoker grate bar area of combustion zone. It is well known that the heat transfer efficiency of a refractory lining is inversely related to its thickness. For example, a refractory having a 2 inch thickness has only 50% of the heat transfer efficiency of the same barrier having a 1 inch depth. Accordingly, the industry has demanded to use refractory lining materials which minimize refractory lining thickness and favor refractory linings as thin as possible.
The metallic waterwall tubes and refractory linings are often installed by hanging them from the ceiling of the building housing the furnace. Since these waterwall tubes and refractory lining can often run about 100 feet tall, the weight of these hanging waterwall tubes and refractory linings presents a safety issue. Accordingly, safety considerations provide further motivation for making refractory barriers as thin as possible.
Although the industry has recognized the need for thin refractory barriers, it also recognizes it cannot reduce the depth of these barriers without usually degrading performance. In particular, it has been found that reducing the depth too much (i.e., down to about 1/2 inch) weakens the strength of the barrier to the point where it cannot withstand the stresses produced by the tubes at high temperatures. Accordingly, the industry routinely uses barriers whose depths are at least about 0.875 to 1.00 inches in minimum cross section.
The MSW industry has developed different types of refractory structures in an effort to simultaneously protect the metallic waterwall tubes while maintaining excellent heat transfer. One such refractory is known as a "monolithic" refractory. A monolithic refractory is produced by gunniting a ceramic material directly onto studded waterwall tubes. However, some monolithic refractories have been known to suffer from low thermal conductivity, low strength, and bonding difficulties which can lead to excessive slag accumulation hampering high thermal conductivity leading to poor efficiency.
Another type of commercial refractory is the "tube tile or block" design. FIG. 2 presents a conventional tube block design. Typically, the tube block is a square or rectangular refractory tile, (typically no more than 8-12 inches in height H by 8-12 inches in width W by 1 inch in depth D), modified on its back face with channels C and ridges R for fitting properly to the waterwall tube design. A refractory wall is built as these tube blocks are assembled in a manner similar to laying bricks, that is, a tube block is set in place, its periphery covered with mortar, and another block is set either atop or beside the first block. This building continues until the desired wall is constructed. The tube block and tube assembly are typically secured by adding a stud S to the membrane M or directly to the waterwall tube passing the stud through a hole H in a ridge R of the tube block, and tightening the stud S by a screw A. See FIG. 3. Typically, the channels of a tube block do not directly contact the metallic tubes they receive. Rather, the channel and tube are bonded together by a mortar interlayer (not shown). Although the mortar provides a good bond between the tubes and the tube block, its own thermal conductivity is poor and so it inhibits the flow of heat from the furnace to the tubes. In general, tube blocks provide the advantages of high strength, better bonding and a higher thermal conductivity than the monolithic designs.
When the conventional tube assembly comprises 3-inch diameter metallic tubes having centers spaced at 4 inch intervals, the single tube block typically has a height of about 77/8 inches, a width of about 77/8 inches, and a depth of 1 inch. This spacing provides an intimate fit between tube blocks (i.e., about 1/8 inch) which reduces the chances of developing an air gap that hinders heat flow between the tubes and the tube block assemblies.
One commercial refractory tube block is the design shown in FIG. 4. This design is similar to the conventional prior art design shown above, except for a groove around the periphery of the block. Although this design possesses the discussed advantages over monolithic barriers, it nonetheless has a depth of at least about 1 inch, and so provides poor heat flow and is heavy.
Another commercial tube block design is the ship-lap design. Originally utilized in circulating fluidized bed boilers, the ship-lap design, shown in FIG. 5, has an interlocking design which prevents small particles (such as sand) from infiltrating the gaps between adjacent tube blocks. However, the interlocking design makes manufacture of the ship-lap design very expensive. Moreover, the depth of a typical ship-lap block is at least about 0,875 inches. Although this generous depth provides insurance against cracks in the tube block, it also significantly inhibits heat flow through the refractory and makes for a very heavy block.
In an effort to improve the thermal conductivity of the tube block designs, U.S. Pat. No. 5,154,139 ("the Johnson patent"), assigned to the Norton Company, disclosed a tube block having a 1/2 inch depth with ribs in its channels. As shown in FIG. 6, when this ribbed tube block is placed against the tube assembly, the ribs contact the tube walls. This direct contact allows heat to bypass the low thermal conductivity mortar and so provides a higher thermal conductivity than the other conventional tube block designs. The slight (i.e., 1/2 inch) depth of this design also enhances its heat conductivity. However, commercial embodiments of the Johnson patent were found to fail in the field. In particular, cracks began to develop in the tube blocks at the point designated as "x" in FIG. 6.
Therefore, there is a need for a refractory tube block which is light and reliable, and has superior heat conductivity.